The architecture of light: architectural lighting design concepts and techniques. A textbook of procedures and practices for the architect, interior designer and lighting designer pdfdrive com

Chapter 18 Lighting Units and Measurements

Up to this point, we have managed to explore the depths of lighting design concept and application without having to burden ourselves with the specifics of lighting science and the systems of units and measurements that go with it. In order to speak knowledgeably about lighting effects and lighting solutions, it is necessary to have a basic understanding of light levels and how they translate visually. In order to engage in this discussion, we must first take a look at lighting science and some of the fundamental building blocks of lighting metrics. This will help us not only communicate our lighting ideas more precisely, but will also allow us to recognize appropriate tools and tactics as they are presented by others.
When we go on to discuss light levels in design, we almost always refer to
the lighting unit of the foot-candle. A foot-candle is a measurement of Illuminance cast onto a surface in a space. The unit of the foot-candle is, indeed, based on the light produced onto an object a “foot” away from a very special candle. We however are not really concerned with what a foot-candle is, but rather what different illuminance levels, expressed in foot-candles, look like.
Let’s start with the basics.
IT’S ALL ABOUT THE LUMEN
Light comes in pieces, or at least we consider that it does for most of our science and study. Scientists call these tiny pieces of light photons, and they exhaustively study all of the ways that photons interact with the physical world.
In lighting science, we are not just concerned with light generically; we
are concerned with how these pieces of light affect human vision. Our visual system is more sensitive to some types of light than others, so we measure a unit of light energy as it affects the sensitivity of the cones and rods in the human eye. We call these modified pieces of light “lumens.” The lumen is the basis of all lighting study, and we can always be safe talking about light in terms of lumens. We study three common ways that lumens interact with the
environment: We study the number of lumens onto or “striking” a surface; called illuminance
We study the number of lumens off of or leaving a surface; called exitance
We study lumens of light leaving a surface or source in a specific direction with a specific density; called luminance
When we talk about lighting effects, we generally talk about light density, and, therefore we talk about how many lumens are acting per area. An unfortunate part of lighting science is that we have come up with different names for the measurements of light interacting in different ways. Figure 18.1 shows the three interactions as lumens of light strike and reflect off of a surface.
Figure 18.1 shows that illuminance and exitance are both measurements
of light density; specifically, a measure of lumens per square-foot. The difference between the two phenomena is simply whether the light is striking a surface (illuminance), or leaving a surface (exitance).
Luminance, however, is a more thorough measurement of how densely
light is leaving in a specific direction.
To completely understand the nuances of these three methods of measuring light, we will elaborate on them here.